This invention relates to an antilock control system and method for controlling vehicle wheel brakes.
When the brakes of a vehicle are applied, a braking force is generated between the wheel and the road surface that is dependent upon various parameters which include the road surface condition and the amount of slip between the wheel and the road surface. This braking force increases as slip increases until a critical value of slip is surpassed. Beyond the critical value of slip, the braking force decreases and the wheel rapidly approaches lockup. Therefore, to achieve stable braking, an antilock control system seeks to operate wheel slip at or near the critical slip value. An antilock control system achieves this objective by detecting an incipient wheel lock condition. Upon detecting an incipient wheel lock condition, the antilock control system releases pressure at the wheel brake to allow recovery from the incipient wheel lock condition. Upon recovery, brake pressure is reapplied. Criteria used to indicate an incipient wheel lock condition includes excessive wheel deceleration and/or excessive wheel slip.
One known antilock control system uses a motor driven pressure modulator in which a DC torque motor drives a piston in a cylinder whose volume is modulated to control the hydraulic brake pressure at the wheel brake. In this system, because of the relationship between motor current, motor torque and motor load represented by the hydraulic brake pressure on the head of the piston, the value of motor current is used as a representation of brake pressure and is controlled to provide control of the brake pressure. In one such system, when an incipient wheel lock condition is sensed, the value of motor current at this time is stored as a representation of the brake pressure producing the maximum braking force coexisting with the critical slip between the wheel and the road surface and the motor current is controlled to quickly retract the piston to release brake pressure to allow recovery from the incipient wheel lock condition. When a recovery from the incipient wheel lock condition is sensed, the motor current is controlled to extend the piston to reapply brake pressure. In reapplying the brake pressure, the pressure is quickly established substantially at the brake pressure producing the maximum braking force by quickly establishing the motor current at a significant fraction of the motor current stored at the time an incipient wheel lock condition was sensed. Thereafter, brake pressure is ramped at a controlled rate which may be a function of wheel slip and acceleration by ramping the motor current in direction applying brake pressure until an incipient wheel lock condition is again sensed after which the cycle is repeated.
In the foregoing form of motor driven pressure modulator, the following dynamic relationships exist: (a) when the brake pressure load on the motor is equal to the motor torque, the motor does not rotate, the piston remains stationary, and the motor current is a measure of the brake pressure and (b) when the brake pressure load on the motor is less than the motor torque, the motor accelerates and rotates at some speed while extending the piston to increase brake pressure. In this latter situation, the speed of the motor is unknown and the motor current is not a true indicator of brake pressure. Accordingly, when a recovery condition from an incipient wheel lockup condition is sensed and motor current is controlled to the significant fraction of the previously stored motor current representing the brake pressure producing the maximum braking force, the motor begins to accelerate to reapply brake pressure. During this period, the relationship between the motor current and brake pressure is unknown so that the value of brake pressure is unknown during this period. It is desirable to begin ramping the motor current when the motor speed in response to the initial reapply current has decreased so that a predictable relationship exists between the motor current and pressure. Thereafter, the motor current may be ramped such as described in copending application Ser. No. 07/531,397, filed May 31, 1990, in the names of Martin A. Hogan et al, and assigned to the assignee of this invention, to provide a desired control of the brake pressure.